Fiber optic sensor arrays of a time division multiplexed (“TDM”) system are often used to measure a change in a parameter, for example, acoustic vibration, fluid pressure variations, acceleration, and magnetic field intensity. The fiber optic sensor array employs a phase generated carrier with a period T to measure the change in the parameter at a given sampling rate. The fiber optic sensor array converts a phase angle associated with the parameter to an amplitude variation on an output pulse of light.
The phase angle is measured through various demodulation techniques of the output pulse. Typical demodulation techniques employ a quadrature component Q and an in-phase component I of the output pulse. The quadrature component Q corresponds to a sine of the phase angle, and the in-phase component I corresponds to a cosine of the phase angle. An arctangent of the ratio Q/I is equal to the phase angle. The magnitude of the change in the parameter can then be calculated from the change in the phase angle.
Calculation of the quadrature component Q and the in-phase component I requires multiple samples of the output pulse at specific intervals of the phase generated carrier. The output pulse is filtered to improve characteristics of the output pulse. A period of the phase generated carrier is significantly longer than a period of the output pulse. The longer period of the phase generated carrier requires the samples to span several output pulses to obtain each required interval of the phase generated carrier. The longer period of the phase generated carrier reduces the sampling rate of the demodulation technique.
High-speed phase generated carriers (e.g., a frequency greater than 1 MHz, or a period less than 1000 nanoseconds) do not permit the precise control of a demodulation phase offset β associated with the phase generated carrier. One shortcoming of the demodulation techniques is that a variation in the demodulation phase offset β from a fixed value reduces the accuracy of the demodulation techniques.
Thus, a need exists for reduced dependency on demodulation phase offsets for demodulation techniques of fiber optic sensor arrays that employ phase generated carriers and filters.